Experimental methods in chemical engineering: Reactors—fluidized beds

Abstract

Industry relies on fluidized beds to synthesize chemicals (acrylonitrile, maleic anhydride, titanium dioxide, vinyl chloride), combust coal, dry powders, and treat waste. Fluidized bed folklore declares that they are hard to scale‐up and the gas phase is backmixed. Commercial failures that disregard standard design criteria around powder management, gas/solids injection, and mixing reinforce this belief. However, engineers select fluidized beds for processes that are impractical with conventional technologies to achieve economies of scale for highly exothermic, endothermic, or explosive reactions, for catalysts that deactivate in seconds (or minutes), and for chemistry that requires multiple dosing cycles. Failures are more frequent for these challenging applications. For this reason, researchers study reaction kinetics in fixed beds despite internal mass transfer limitations and axial and radial temperature and concentration gradients. Fluidized bed hydrodynamics vary with powder properties (particle diameter, size distribution, density, sphericity), operating conditions (gas density, viscosity, temperature, pressure), reactor geometry (diameter, height, mass, grid geometry). The minimum fluidization velocity (Umf) is a property that identifies the transition from the fixed bed regime to the fluidized bed regime and equals the gas velocity at which the upward drag force equals the weight of the powder. At the experimental scale, fluidized beds operate isothermally, solids are completely backmixed, and the gas phase is close to plug flow (Ug<-0.25em3Umf). Here, we describe the relationship between powder properties and fluidization quality, list experimental techniques, describe recent applications, and gas phase hydrodynamics and uncertainties.

Experimental methods in chemical engineering: Residence time distribution—RTD

“…For moderate and low degrees of dispersion, we solve the analytical equation to calculate the Peclet number . The system is close to plug flow when N Pe > 50 but even values in the vicinity of 20 are good approximations to most reactor systems, particularly when conversion is less than 90% . For small deviations from plug flow, N Pe > 100, the analytical solution is: and in dimensionless form: …”

“…Each test was performed with a fluid catalytic catalyst (FCC) belonging to Geldart group A classification with a sphericity close to one ( ϕ ≈ 1) (Figure ) and its minimum fluidization velocity ( U mf ) about 2.2 mm · s −1 (Table ) . The reactor operated at 10, 20 and 60 mm · s −1 , which represents 4.5, 9 or 27 times U mf .…”

“…Scanning electron microscopy (SEM) image of FCC. Most of the particles are spheroids, but some are fractured, and others form agglomerates …”

“…[26] The system is close to plug flow when N Pe > 50 [41] but even values in the vicinity of 20 are good approximations to most reactor systems, particularly when conversion is less than 90%. [42] For small deviations from plug flow, N Pe > 100, the analytical solution is [35] :…”

Issue Highlights

“…At gas velocities near the minimum fluidization velocity ( U mf ), the gas flow regime approximates plug flow with n CSTR > 15, but it drops off to below 5 at 6 U mf . Pressure drop, optical probes, and image velocimetry are main the measuring techniques …”